Test methods for optical fiber coatings

Oct. 1, 1997

Test methods for optical fiber coatings

By PRIYA TABADDOR

The Telecommunications Industry Association (tia) Working Group FO-6.6.7 on Fiber Coatings, chaired by Eric Urruti of Corning Inc. in Corning, NY, addresses issues related to coating characteristics and evaluation.

UV-curable protective coatings for optical fibers require a broad range of performance and processing characteristics. These polymer coatings affect both the optical and mechanical properties of the fiber.

Standards bodies have been trying for years to devise a test that would rank order the effectiveness of various coating systems in minimizing microbending loss. In the "basketweave" test, loss is measured for a coated fiber wound in multiple overlapping turns on a cylindrical drum (fiber-optic test procedure fotp-112). In the "wire mesh" method, a straight length of coated fiber is sandwiched between a wire screen (typically 70 mesh) and a rubber pad. The International Electrotechnical Commission`s (iec) method 793-1-C3B requires winding at least 400 m of fiber in a single layer onto a drum whose surface has been covered with sandpaper. An alternative method called for the use of a drum whose diameter could be expanded continuously (iec`s method 793-1-C3A and tia`s fotp-110).

Several methods have been evaluated, but FO-6.6.7 has concluded that none are suitable for rank ordering coatings. In addition, these tests have been deemed "unspecifiable" in trade and commerce, and will be redefined as informative test methods (itms) rather than fotps.

tia has generated a number of fotps for measuring coating geometry: the end-view method (fotp-55B), the side-view method (fotp-173B), the side-view gray-scale method (fotp-119), and the side-view ray-traced method (fotp-163). These will be consolidated into a single document. A round robin involving these methods was coordinated by the National Institute of Standards and Technology (nist). A final report, nist TechNote 1378, is available from nist`s Tim Drapela at (303) 497-5858.

Round robins have shown the value of using a standard reference material (srm) that can be used to calibrate test sets. nist is offering for sale srm #2521, a glass cylinder whose refractive index approximates that of a polymer coating (about 1.52) and whose diameter is certified by nist to ۪.1 micron. Further information is available from nist`s srm program office at (301) 975-6776.

While coating robustness is critical to ensure fiber integrity during handling, it is important to be able to mechanically remove the coating for splicing operations. fotp-178 measures the strip force required to mechanically remove coatings from fibers, and TSB-62-118 is an itm for removing coatings during ribbon stripping. Single fiber strip force is dependent on both the modulus and elongation of the secondary coating. Ribbon stripping characteristics, on the other hand, are controlled not only by the properties of the secondary coating, but also by adhesion between the primary coating and glass. An itm is in preparation for the fiber pull-out force, typically used to measure interfacial adhesion, where a tensile force is imposed on the specimen to separate the primary coating from the glass.

Coating transparency becomes an important issue when local-injection and local-detection devices, fiber identifiers, and optical taps are used. fotp-106 measures the absorbance of the coating material (prepared as a thin film with or without pigments) in a spectrophotometer. q

Priya Tabaddor is a member of technical staff at Bell Laboratories, and represents Lucent Technologies on coating standards committees. Tel: (770) 798-2962, fax: (770) 798-4655, and e-mail: [email protected].

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